Not applicable.
1. Field of the Invention
This invention relates to coating systems suitable for protecting components exposed to high-temperature environments, such as the hot gas flow path through a gas turbine engine. More particularly, this invention is directed to thermal barrier coating (TBC) systems and environmental barrier coating (EBC) systems characterized by a low coefficient of thermal conductivity, and a method by which multilayer TBC""s and EBC""s can be constructed to maintain a low thermal conductivity throughout numerous thermal cycles.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. For this reason, the use of thermal barrier coatings (TBC) on components such as combustors, high pressure turbine (HPT) blades and vanes has increased in commercial as well as military gas turbine engines. The thermal insulation of a TBC enables components formed of superalloys and other high temperature materials to survive higher operating temperatures, increases component durability, and improves engine reliability.
TBC is typically a thermal-insulating ceramic material deposited on an environmentally-protective bond coat to form what is termed a TBC system. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and oxidation-resistant diffusion coatings such as diffusion aluminides that contain aluminum intermetallics. Binary yttria-stabilized zirconia (YSZ) is widely used as the thermal insulating layer of TBC systems because of its high temperature capability, low thermal conductivity, and relative ease of deposition by air plasma spraying (APS), flame spraying and physical vapor deposition (PVD) techniques. TBC""s formed by these methods have a lower thermal conductivity than a dense ceramic of the same composition as a result of the presence of microstructural defects and pores at and between grain boundaries of the TBC microstructure. TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.).
While significant advances in high temperature capabilities have been achieved through advancements in superalloy and TBC materials, alternative materials have been proposed. For example, materials containing silicon, such as those with silicon carbide (SiC) as a matrix material or a reinforcing material, are currently being considered for components used in high temperature applications, such as combustor and other hot section components of gas turbine engines. Similar to superalloy components, a thermal-insulating coating over the Si-containing material is often desirable or necessary to reduce the operating temperature and thermal gradient through the material. It is also often desirable or necessary to provide a coating that can provide environmental protection by inhibiting the major mechanism for degradation of silicon carbide in a corrosive environment, namely, the formation of volatile silicon monoxide (SiO) and silicon hydroxide (Si(OH)4) products. Consequently, coatings for components formed of Si-containing materials often have a dual function, serving as a thermal barrier and simultaneously providing protection from the environment. Coating systems having this dual function will be referred to herein as an environmental barrier coating (EBC) system. Additional requirements of EBC systems for SiC-containing materials include low activity of silica (SiO2) in its composition, a coefficient of thermal expansion (CTE) compatible with the SiC-containing material, low permeability for oxidants, and chemical compatibility with SiC and silica scale.
In order for a TBC or EBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC or EBC has and maintains a low thermal conductivity throughout the life of the component, including high temperature excursions. However, the thermal conductivities of TBC and EBC materials such as YSZ are known to increase over time when subjected to the operating environment of a gas turbine engine. As a result, TBC""s and EBC""s for gas turbine engine components are often deposited to a greater thickness than would otherwise be necessary. Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow. Both of these solutions are undesirable for reasons relating to cost, component life and engine efficiency.
In view of the above, it can be appreciated that further improvements in coating technology are desirable, particularly as TBC""s and EBC""s are employed to thermally insulate components intended for more demanding engine designs. A coating with lower thermal conductivity would allow for higher component surface temperatures or reduced coating thickness for the same surface temperature. Reduced coating thickness, especially in applications like combustors which require relatively thick TBC""s and EBC""s, would result in a significant cost reduction as well as weight benefit.
The present invention provides a coating material having a low thermal conductivity, and a method by which the low thermal conductivity of the coating material is maintained through the intentional but controlled development of cracks within the coating material. According to a preferred aspect of the invention, the coating material serves as a thermal-insulating layer of a TBC or EBC, and is a mixture of two or more materials with different coefficients of thermal expansion (CTE). The materials of the thermal-insulating layer are selected and combined so that a low thermal conductivity is maintained as the result of microcracks developing and propagating from interfaces between the materials when the thermal-insulating layer is subjected to heating and cooling cycles. According to the invention, the thermal-insulating layer is formed so that the microcracks propagate in a direction transverse to the direction of heat transfer through the TBC/EBC, e.g., to the surface of the component, and form and are reformed in the thermal-insulating layer with each thermal cycle so that microcracks are continuously present in the thermal-insulating layer to provide a physical barrier to heat transfer to the component.
TBC""s and EBC""s in accordance with the present invention can have a significantly lower thermal conductivity than those of conventional YSZ, and are particularly suitable for thermally insulating components intended for demanding applications, including advanced gas turbine engines in which higher component surface temperatures are required. Alternatively, the lower thermal conductivity of the thermal-insulating layer allows for reduced coating thicknesses for the same surface temperature, resulting in a significant cost reduction as well as weight benefit.